1. Field of the Invention
This invention relates in general to a piezo-resistive silicon membrane pressure sensor in which an overload member is mounted between the membrane chip and the carrier chip so as to protect the membrane.
2. Description of Related Art
The overload strength of piezo-resistive silicon membrane pressure sensors can amount to approximately twenty times the nominal pressure in the low pressure range which is too low for many applications. When measuring pressure differences in the millibar range in high pressure systems for example, the full system pressure can act on the pressure sensor as a differential pressure as a consequence of a local pressure drop for example, due to a rupture in the line. The overload factors of about 1000 and above are therefore required in actual practice. An additional overload protection which is effective independently of the direction of the pressure, in other words, independently of whether the overpressure acts on the front or on the backside of the membrane of the pressure sensor must therefore be realized in a pressure sensor for such applications.
Up to the present time silicon pressure sensors were usually protected against destruction by using complicated mechanical designs which have metal membranes or overload valves. Critical advantages of silicon pressure sensors however, are therefore lost, namely, the advantages of small dimensions and low weight and low cost.
The small inertia of silicon pressure sensors represents an additional problem because silicon pressure sensors have a natural frequency of approximately 50kH. The silicon pressure sensors therefore must be more inert with damping since the mechanical protective measures would otherwise fail when extremely short pressure surges occur. On the other hand, the quick acquisition of the pressure is exactly what is needed in many applications so that a solution without damping is desirable.
Prior to the above-mentioned macro-mechanical solutions with metal membranes or overload valves, a micro-mechanical solution was known providing an integrated protection against overload from the backside of the membrane. This known integrated overload protection however is effective only for overloads in one direction.
See also U.S. Pat. No. 4,761,210, U.S. Pat. No. 4,028,945, British Patent Application 2,069,703, the European Patent Application 0,217,071, German 77 03 675 and Japanese Patent Abstracts Volume 7, No. 270, Dec. 2, 1983 No. 58-151536.